Posts Tagged ‘root server’

Disable VNC on KVM Virtual Machine without VM restart / How to Change VNC listen address

Monday, February 28th, 2022

disable-vnc-port-listener-on-a-KVM-ran-virtual-machine-virsh-libvirt-libvirt-architecture-design

Say you have recently run a new KVM Virtual machine, have connected via VNC on lets say the default tcp port 5900 
installed a brand new Linux OS using a VNC client to connect, such as:
TightVNC / RealVNC if connecting from Windows Client machine or Vncviewer / Remmina if connecting from Linux / BSD  and now 
you want to turn off the VNC VM listener server either for security reasons to make sure some script kiddie random scanner did not manage to connect and take control over your VM or just because, you will be only further using the new configured VM only via SSH console sessions as they call it in modern times to make a buziness buzz out of it a headless UNIX server (server machines connected a network without a Physical monitor attached to it).


The question comes then how can be the KVM VNC listener on TCP port 5900 be completely disabled?

One way of course is to filter out with a firewall 5900 completely either on a Switch Level (lets say on a Cisco equipment catalist in front of the machine) or the worst solution to  locally filter directly on the server with firewalld or iptables chain rules.
 

1. Disable KVM VNC Port listener via VIRSH VM XML edit

The better way of course  is to completely disable the VNC using KVM, that is possible through the virsh command interface.
By editing the XML Virtual Machine configuration and finding the line about vnc confiuguration with:

root@server:/kvm/disk# virsh edit pcfreakweb
Domain pcfreakweb XML configuration not changed.

like:

<graphics type='vnc' port='5900' autoport='yes' listen='0.0.0.0'>
      <listen type='address' address='0.0.0.0'/>


and set value to undefined:

port='-1'


virsh-KVM-disable-VNC-port-listener-virsh-xml-edit-screenshot

Modifying the XML however will require you to reboot the Virtual Machine for which XML was editted. This might be not possible
if you have a running production server already configured with Apache / Proxy / PostgreSQL / Mail or any other Internet public service.

2. Disable VNC KVM TCP port 5900 to a dynamic running VM without a machine reboot


Thus if you want to remove the KVM VNC Port Listener on 5900 without a VM shutdown / reboot you can do it via KVM's virsh client interface.

root@server:/kvm/disk# virsh
Welcome to virsh, the virtualization interactive terminal.

Type:  'help' for help with commands
       'quit' to quit

virsh # qemu-monitor-command pcfreakweb –hmp change  vnc none

 

The virsh management user interface client, can do pretty much more of real time VM changes, it is really useful to use it if you have KVM Hypervisor hosts with 10+ Virtual machines and it if you have to deal with KVM machines on daily, do specific changes to the VMs on how VM networks are configured, information on HV hardware, configure / reconfigure storage volumes to VMs etc, take some time to play with it 🙂

List and fix failed systemd failed services after Linux OS upgrade and how to get full info about systemd service from jorunal log

Friday, February 25th, 2022

systemd-logo-unix-linux-list-failed-systemd-services

I have recently upgraded a number of machines from Debian 10 Buster to Debian 11 Bullseye. The update as always has some issues on some machines, such as problem with package dependencies, changing a number of external package repositories etc. to match che Bullseye deb packages. On some machines the update was less painful on others but the overall line was that most of the machines after the update ended up with one or more failed systemd services. It could be that some of the machines has already had this failed services present and I never checked them from the previous time update from Debian 9 -> Debian 10 or just some mess I've left behind in the hurry when doing software installation in the past. This doesn't matter anyways the fact was that I had to deal to a number of systemctl services which I managed to track by the Failed service mesage on system boot on one of the physical machines and on the OpenXen VTY Console the rest of Virtual Machines after update had some Failed messages. Thus I've spend some good amount of time like an overall of a day or two fixing strange failed services. This is how this small article was born in attempt to help sysadmins or any home Linux desktop users, who has updated his Debian Linux / Ubuntu or any other deb based distribution but due to the chaotic nature of Linux has ended with same strange Failed services and look for a way to find the source of the failures and get rid of the problems. 
Systemd is a very complicated system and in my many sysadmin opinion it makes more problems than it solves, but okay for today's people's megalomania mindset it matches well.

Systemd_components-systemd-journalctl-cgroups-loginctl-nspawn-analyze.svg

 

1. Check the journal for errors, running service irregularities and so on
 

First thing to do to track for errors, right after the update is to take some minutes and closely check,, the journalctl for any strange errors, even on well maintained Unix machines, this journal log would bring you to a problem that is not fatal but still some process or stuff is malfunctioning in the background that you would like to solve:
 

root@pcfreak:~# journalctl -x
Jan 10 10:10:01 pcfreak CRON[17887]: pam_unix(cron:session): session closed for user root
Jan 10 10:10:01 pcfreak audit[17887]: USER_END pid=17887 uid=0 auid=0 ses=340858 subj==unconfined msg='op=PAM:session_close grantors=pam_loginuid,pam_env,pam_env,pam_permit>
Jan 10 10:10:01 pcfreak audit[17888]: CRED_DISP pid=17888 uid=0 auid=0 ses=340860 subj==unconfined msg='op=PAM:setcred grantors=pam_permit acct="root" exe="/usr/sbin/cron" >
Jan 10 10:10:01 pcfreak CRON[17888]: pam_unix(cron:session): session closed for user root
Jan 10 10:10:01 pcfreak audit[17888]: USER_END pid=17888 uid=0 auid=0 ses=340860 subj==unconfined msg='op=PAM:session_close grantors=pam_loginuid,pam_env,pam_env,pam_permit>
Jan 10 10:10:01 pcfreak audit[17884]: CRED_DISP pid=17884 uid=0 auid=0 ses=340855 subj==unconfined msg='op=PAM:setcred grantors=pam_permit acct="root" exe="/usr/sbin/cron" >
Jan 10 10:10:01 pcfreak CRON[17884]: pam_unix(cron:session): session closed for user root
Jan 10 10:10:01 pcfreak audit[17884]: USER_END pid=17884 uid=0 auid=0 ses=340855 subj==unconfined msg='op=PAM:session_close grantors=pam_loginuid,pam_env,pam_env,pam_permit>
Jan 10 10:10:01 pcfreak audit[17886]: CRED_DISP pid=17886 uid=0 auid=33 ses=340859 subj==unconfined msg='op=PAM:setcred grantors=pam_permit acct="www-data" exe="/usr/sbin/c>
Jan 10 10:10:01 pcfreak CRON[17886]: pam_unix(cron:session): session closed for user www-data
Jan 10 10:10:01 pcfreak audit[17886]: USER_END pid=17886 uid=0 auid=33 ses=340859 subj==unconfined msg='op=PAM:session_close grantors=pam_loginuid,pam_env,pam_env,pam_permi>
Jan 10 10:10:08 pcfreak NetworkManager[696]:  [1641802208.0899] device (eth1): carrier: link connected
Jan 10 10:10:08 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Up – 100Mbps/Full – flow control rx/tx
Jan 10 10:10:08 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Down
Jan 10 10:10:19 pcfreak NetworkManager[696]:
 [1641802219.7920] device (eth1): carrier: link connected
Jan 10 10:10:19 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Up – 100Mbps/Full – flow control rx/tx
Jan 10 10:10:20 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Down
Jan 10 10:10:22 pcfreak NetworkManager[696]:
 [1641802222.2772] device (eth1): carrier: link connected
Jan 10 10:10:22 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Up – 100Mbps/Full – flow control rx/tx
Jan 10 10:10:23 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Down
Jan 10 10:10:33 pcfreak sshd[18142]: Unable to negotiate with 66.212.17.162 port 19255: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diff>
Jan 10 10:10:41 pcfreak NetworkManager[696]:
 [1641802241.0186] device (eth1): carrier: link connected
Jan 10 10:10:41 pcfreak kernel: r8169 0000:03:00.0 eth1: Link is Up – 100Mbps/Full – flow control rx/tx

If you want to only check latest journal log messages use the -x -e (pager catalog) opts

root@pcfreak;~# journalctl -xe

Feb 25 13:08:29 pcfreak audit[2284920]: USER_LOGIN pid=2284920 uid=0 auid=4294967295 ses=4294967295 subj==unconfined msg='op=login acct=28696E76616C>
Feb 25 13:08:29 pcfreak sshd[2284920]: Received disconnect from 177.87.57.145 port 40927:11: Bye Bye [preauth]
Feb 25 13:08:29 pcfreak sshd[2284920]: Disconnected from invalid user ubuntuuser 177.87.57.145 port 40927 [preauth]

Next thing to after the update was to get a list of failed service only.


2. List all systemd failed check services which was supposed to be running

root@pcfreak:/root # systemctl list-units | grep -i failed
● certbot.service                                                                                                       loaded failed failed    Certbot
● logrotate.service                                                                                                     loaded failed failed    Rotate log files
● maldet.service                                                                                                        loaded failed failed    LSB: Start/stop maldet in monitor mode
● named.service                                                                                                         loaded failed failed    BIND Domain Name Server


Alternative way is with the –failed option

hipo@jeremiah:~$ systemctl list-units –failed
  UNIT                        LOAD   ACTIVE SUB    DESCRIPTION
● haproxy.service             loaded failed failed HAProxy Load Balancer
● libvirt-guests.service      loaded failed failed Suspend/Resume Running libvirt Guests
● libvirtd.service            loaded failed failed Virtualization daemon
● nvidia-persistenced.service loaded failed failed NVIDIA Persistence Daemon
● sqwebmail.service           masked failed failed sqwebmail.service
● tpm2-abrmd.service          loaded failed failed TPM2 Access Broker and Resource Management Daemon
● wd_keepalive.service        loaded failed failed LSB: Start watchdog keepalive daemon

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.
7 loaded units listed.

 

root@jeremiah:/etc/apt/sources.list.d#  systemctl list-units –failed
  UNIT                        LOAD   ACTIVE SUB    DESCRIPTION
● haproxy.service             loaded failed failed HAProxy Load Balancer
● libvirt-guests.service      loaded failed failed Suspend/Resume Running libvirt Guests
● libvirtd.service            loaded failed failed Virtualization daemon
● nvidia-persistenced.service loaded failed failed NVIDIA Persistence Daemon
● sqwebmail.service           masked failed failed sqwebmail.service
● tpm2-abrmd.service          loaded failed failed TPM2 Access Broker and Resource Management Daemon
● wd_keepalive.service        loaded failed failed LSB: Start watchdog keepalive daemon


To get a full list of objects of systemctl you can pass as state:
 

# systemctl –state=help
Full list of possible load states to pass is here
Show service properties


Check whether a service is failed or has other status and check default set systemd variables for it.

root@jeremiah~:# systemctl is-failed vboxweb.service
inactive

# systemctl show haproxy
Type=notify
Restart=always
NotifyAccess=main
RestartUSec=100ms
TimeoutStartUSec=1min 30s
TimeoutStopUSec=1min 30s
TimeoutAbortUSec=1min 30s
TimeoutStartFailureMode=terminate
TimeoutStopFailureMode=terminate
RuntimeMaxUSec=infinity
WatchdogUSec=0
WatchdogTimestampMonotonic=0
RootDirectoryStartOnly=no
RemainAfterExit=no
GuessMainPID=yes
SuccessExitStatus=143
MainPID=304858
ControlPID=0
FileDescriptorStoreMax=0
NFileDescriptorStore=0
StatusErrno=0
Result=success
ReloadResult=success
CleanResult=success

Full output of the above command is dumped in show_systemctl_properties.txt


3. List all running systemd services for a better overview on what's going on on machine
 

To get a list of all properly systemd loaded services you can use –state running.

hipo@jeremiah:~$ systemctl list-units –state running|head -n 10
  UNIT                              LOAD   ACTIVE SUB     DESCRIPTION
  proc-sys-fs-binfmt_misc.automount loaded active running Arbitrary Executable File Formats File System Automount Point
  cups.path                         loaded active running CUPS Scheduler
  init.scope                        loaded active running System and Service Manager
  session-2.scope                   loaded active running Session 2 of user hipo
  accounts-daemon.service           loaded active running Accounts Service
  anydesk.service                   loaded active running AnyDesk
  apache-htcacheclean.service       loaded active running Disk Cache Cleaning Daemon for Apache HTTP Server
  apache2.service                   loaded active running The Apache HTTP Server
  avahi-daemon.service              loaded active running Avahi mDNS/DNS-SD Stack

 

It is useful thing is to list all unit-files configured in systemd and their state, you can do it with:

 


root@pcfreak:~# systemctl list-unit-files
UNIT FILE                                                                 STATE           VENDOR PRESET
proc-sys-fs-binfmt_misc.automount                                         static          –            
-.mount                                                                   generated       –            
backups.mount                                                             generated       –            
dev-hugepages.mount                                                       static          –            
dev-mqueue.mount                                                          static          –            
media-cdrom0.mount                                                        generated       –            
mnt-sda1.mount                                                            generated       –            
proc-fs-nfsd.mount                                                        static          –            
proc-sys-fs-binfmt_misc.mount                                             disabled        disabled     
run-rpc_pipefs.mount                                                      static          –            
sys-fs-fuse-connections.mount                                             static          –            
sys-kernel-config.mount                                                   static          –            
sys-kernel-debug.mount                                                    static          –            
sys-kernel-tracing.mount                                                  static          –            
var-www.mount                                                             generated       –            
acpid.path                                                                masked          enabled      
cups.path                                                                 enabled         enabled      

 

 


root@pcfreak:~# systemctl list-units –type service –all
  UNIT                                   LOAD      ACTIVE   SUB     DESCRIPTION
  accounts-daemon.service                loaded    inactive dead    Accounts Service
  acct.service                           loaded    active   exited  Kernel process accounting
● alsa-restore.service                   not-found inactive dead    alsa-restore.service
● alsa-state.service                     not-found inactive dead    alsa-state.service
  apache2.service                        loaded    active   running The Apache HTTP Server
● apparmor.service                       not-found inactive dead    apparmor.service
  apt-daily-upgrade.service              loaded    inactive dead    Daily apt upgrade and clean activities
 apt-daily.service                      loaded    inactive dead    Daily apt download activities
  atd.service                            loaded    active   running Deferred execution scheduler
  auditd.service                         loaded    active   running Security Auditing Service
  auth-rpcgss-module.service             loaded    inactive dead    Kernel Module supporting RPCSEC_GSS
  avahi-daemon.service                   loaded    active   running Avahi mDNS/DNS-SD Stack
  certbot.service                        loaded    inactive dead    Certbot
  clamav-daemon.service                  loaded    active   running Clam AntiVirus userspace daemon
  clamav-freshclam.service               loaded    active   running ClamAV virus database updater
..

 


linux-systemd-components-diagram-linux-kernel-system-targets-systemd-libraries-daemons

 

4. Finding out more on why a systemd configured service has failed


Usually getting info about failed systemd service is done with systemctl status servicename.service
However, in case of troubles with service unable to start to get more info about why a service has failed with (-l) or (–full) options


root@pcfreak:~# systemctl -l status logrotate.service
● logrotate.service – Rotate log files
     Loaded: loaded (/lib/systemd/system/logrotate.service; static)
     Active: failed (Result: exit-code) since Fri 2022-02-25 00:00:06 EET; 13h ago
TriggeredBy: ● logrotate.timer
       Docs: man:logrotate(8)
             man:logrotate.conf(5)
    Process: 2045320 ExecStart=/usr/sbin/logrotate /etc/logrotate.conf (code=exited, status=1/FAILURE)
   Main PID: 2045320 (code=exited, status=1/FAILURE)
        CPU: 2.479s

Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| WARNING: For now we will assume you meant to write /32
Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| ERROR: '0.0.0.0/0.0.0.0' needs to be replaced by the term 'all'.
Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| SECURITY NOTICE: Overriding config setting. Using 'all' instead.
Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| WARNING: (B) '::/0' is a subnetwork of (A) '::/0'
Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| WARNING: because of this '::/0' is ignored to keep splay tree searching predictable
Feb 25 00:00:06 pcfreak logrotate[2045577]: 2022/02/25 00:00:06| WARNING: You should probably remove '::/0' from the ACL named 'all'
Feb 25 00:00:06 pcfreak systemd[1]: logrotate.service: Main process exited, code=exited, status=1/FAILURE
Feb 25 00:00:06 pcfreak systemd[1]: logrotate.service: Failed with result 'exit-code'.
Feb 25 00:00:06 pcfreak systemd[1]: Failed to start Rotate log files.
Feb 25 00:00:06 pcfreak systemd[1]: logrotate.service: Consumed 2.479s CPU time.


systemctl -l however is providing only the last log from message a started / stopped or whatever status service has generated. Sometimes systemctl -l servicename.service is showing incomplete the splitted error message as there is a limitation of line numbers on the console, see below

 

root@pcfreak:~# systemctl status -l certbot.service
● certbot.service – Certbot
     Loaded: loaded (/lib/systemd/system/certbot.service; static)
     Active: failed (Result: exit-code) since Fri 2022-02-25 09:28:33 EET; 4h 0min ago
TriggeredBy: ● certbot.timer
       Docs: file:///usr/share/doc/python-certbot-doc/html/index.html
             https://certbot.eff.org/docs
    Process: 290017 ExecStart=/usr/bin/certbot -q renew (code=exited, status=1/FAILURE)
   Main PID: 290017 (code=exited, status=1/FAILURE)
        CPU: 9.771s

Feb 25 09:28:33 pcfrxen certbot[290017]: The error was: PluginError('An authentication script must be provided with –manual-auth-hook when using th>
Feb 25 09:28:33 pcfrxen certbot[290017]: All renewals failed. The following certificates could not be renewed:
Feb 25 09:28:33 pcfrxen certbot[290017]:   /etc/letsencrypt/live/mail.pcfreak.org-0003/fullchain.pem (failure)
Feb 25 09:28:33 pcfrxen certbot[290017]:   /etc/letsencrypt/live/www.eforia.bg-0005/fullchain.pem (failure)
Feb 25 09:28:33 pcfrxen certbot[290017]:   /etc/letsencrypt/live/zabbix.pc-freak.net/fullchain.pem (failure)
Feb 25 09:28:33 pcfrxen certbot[290017]: 3 renew failure(s), 5 parse failure(s)
Feb 25 09:28:33 pcfrxen systemd[1]: certbot.service: Main process exited, code=exited, status=1/FAILURE
Feb 25 09:28:33 pcfrxen systemd[1]: certbot.service: Failed with result 'exit-code'.
Feb 25 09:28:33 pcfrxen systemd[1]: Failed to start Certbot.
Feb 25 09:28:33 pcfrxen systemd[1]: certbot.service: Consumed 9.771s CPU time.

 

5. Get a complete log of journal to make sure everything configured on server host runs as it should

Thus to get more complete list of the message and be able to later google and look if has come with a solution on the internet  use:

root@pcfrxen:~#  journalctl –catalog –unit=certbot

— Journal begins at Sat 2022-01-22 21:14:05 EET, ends at Fri 2022-02-25 13:32:01 EET. —
Jan 23 09:58:18 pcfrxen systemd[1]: Starting Certbot…
░░ Subject: A start job for unit certbot.service has begun execution
░░ Defined-By: systemd
░░ Support: https://www.debian.org/support
░░ 
░░ A start job for unit certbot.service has begun execution.
░░ 
░░ The job identifier is 5754.
Jan 23 09:58:20 pcfrxen certbot[124996]: Traceback (most recent call last):
Jan 23 09:58:20 pcfrxen certbot[124996]:   File "/usr/lib/python3/dist-packages/certbot/_internal/renewal.py", line 71, in _reconstitute
Jan 23 09:58:20 pcfrxen certbot[124996]:     renewal_candidate = storage.RenewableCert(full_path, config)
Jan 23 09:58:20 pcfrxen certbot[124996]:   File "/usr/lib/python3/dist-packages/certbot/_internal/storage.py", line 471, in __init__
Jan 23 09:58:20 pcfrxen certbot[124996]:     self._check_symlinks()
Jan 23 09:58:20 pcfrxen certbot[124996]:   File "/usr/lib/python3/dist-packages/certbot/_internal/storage.py", line 537, in _check_symlinks

root@server:~# journalctl –catalog –unit=certbot|grep -i pluginerror|tail -1
Feb 25 09:28:33 pcfrxen certbot[290017]: The error was: PluginError('An authentication script must be provided with –manual-auth-hook when using the manual plugin non-interactively.')


Or if you want to list and read only the last messages in the journal log regarding a service

root@server:~# journalctl –catalog –pager-end –unit=certbot


If you have disabled a failed service because you don't need it to run at all on the machine with:

root@rhel:~# systemctl stop rngd.service
root@rhel:~# systemctl disable rngd.service

And you want to clear up any failed service information that is kept in the systemctl service log you can do it with:
 

root@rhel:~# systemctl reset-failed

Another useful systemctl option is cat, you can use it to easily list a service it is useful to quickly check what is a service, an actual shortcut to save you from giving a full path to the service e.g. cat /lib/systemd/system/certbot.service

root@server:~# systemctl cat certbot
# /lib/systemd/system/certbot.service
[Unit]
Description=Certbot
Documentation=file:///usr/share/doc/python-certbot-doc/html/index.html
Documentation=https://certbot.eff.org/docs
[Service]
Type=oneshot
ExecStart=/usr/bin/certbot -q renew
PrivateTmp=true


After failed SystemD services are fixed, it is best to reboot the machine and check put some more time to inspect rawly the complete journal log to make sure, no error  was left behind.


Closure
 

As you can see updating a machine from a major to a major version even if you follow the official documentation and you have plenty of experience is always more or a less a pain in the ass, which can eat up much of your time banging your head solving problems with failed daemons issues with /etc/rc.local (which I have faced becase of #/bin/sh -e (which would make /etc/rc.local) to immediately quit if any error from command $? returns different from 0 etc.. The  logical questions comes then;
1. Is it really worthy to update at all regularly, especially if you don't know of a famous major Vulnerability 🙂 ?
2. Or is it worthy to update from OS major release to OS major release at all?  
3. Or should you only try to patch the service that is exposed to an external reachable computer network or the internet only and still the the same OS release until End of Life (LTS = Long Term Support) as called in Debian or  End Of Life  (EOL) Cycle as called in RPM based distros the period until the OS major release your software distro has official security patches is reached.

Anyone could take any approach but for my own managed systems small network at home my practice was always to try to keep up2date everything every 3 or 6 months maximum. This has caused me multiple days of irritation and stress and perhaps many white hairs and spend nerves on shit.


4. Based on the company where I'm employed the better strategy is to patch to the EOL is still offered and keep the rule First Things First (FTF), once the EOL is reached, just make a copy of all servers data and configuration to external Data storage, bring up a new Physical or VM and migrate the services.
Test after the migration all works as expected if all is as it should be change the DNS records or Leading Infrastructure Proxies whatever to point to the new service and that's it! Yes it is true that migration based on a full OS reinstall is more time consuming and requires much more planning, but usually the result is much more expected, plus it is much less stressful for the guy doing the job.

Update reverse sshd config with cronjob to revert if sshd reload issues

Friday, February 12th, 2021

Update-reverse-sshd-config-with-cronjob-to-revert-if-sshd-reload-issues

Say you're doing ssh hardening modifying /etc/ssh/sshd_config for better system security or just changing options in sshd due to some requirements. But you follow the wrong guide and you placed some ssh variable which is working normally on newer SSH versions ssh OpenSSH_8.0p1 / or 7 but the options are applied on older SSH server and due to that restarting sshd via /etc/init.d/… or systemctl restart sshd cuts your access to remote server located in a DC and not attached to Admin LAN port, and does not have a working ILO or IDRAC configured and you have to wait for a couple of hours for some Support to go to the server Room / Rack / line location to have access to a Linux physical tty console and fix it by reverting the last changes you made to sshd and restarting.

Thus logical question comes what can you do to assure yourself you would not cut your network access to remote machine after modifying OpenSSHD and normal SSHD restart?

There is an old trick, I'm using for years now but perhaps if you're just starting with Linux as a novice system administrator or a server support guy you would not know it, it is as simple as setting a cron job for some minutes to periodically overwrite the sshd configuration with a copy of the old working version of sshd before modification.

Here is this nice nify trick which saved me headache of call on technical support line to ValueWeb when I was administering some old Linux servers back in the 2000s

root@server:~# crontab -u root -e

# create /etc/ssh/sshd_config backup file
cp -rpf /etc/ssh/sshd_config /etc/ssh/sshd_config_$(date +%d-%m-%y)
# add to cronjob to execute every 15 minutes and ovewrite sshd with the working version just in case
*/15 * * * * /bin/cp -rpf /etc/ssh/sshd_config_$(date +%d-%m-%y) /etc/ssh/sshd_config && /bin/systemctl restart sshd
# restart sshd 
cp -rpf /etc/ssh/sshd_config_$(date +%d-%m-%y) /etc/ssh/sshd_config && /bin/systemctl restart sshd


Copy paste above cron definitions and leave them on for some time. Do the /etc/ssh/sshd_config modifications and once you're done restart sshd by lets say

root@server:~#  killall -HUP sshd 


If the ssh connectivity continues to work edit the cron job again and delete all lines and save again.
If you're not feeling confortable with vim as a text editor (in case you're a complete newbie and you don't know) how to get out of vim. Before doing all little steps you can do on the shell with  export EDITOR=nano or export EDITOR=mcedit cmds,this will change the default text editor on the shell. 

Hope this helps someone… Enjoy 🙂

Check linux install date / How do I find out how long a Linux server OS was installed?

Wednesday, March 30th, 2016

linux-check-install-date-howto-commands-on-debian-and-fedora-tux_the_linux_penguin_by_hello

To find out the Linux install date, there is no one single solution according to the Linux distribution type and version, there are some common ways to get the Linux OS install age.
Perhaps the most popular way to get the OS installation date and time is to check out when the root filesystem ( / ) was created, this can be done with tune2fs command

 

server:~# tune2fs -l /dev/sda1 | grep 'Filesystem created:'
Filesystem created:       Thu Sep  6 21:44:22 2012

 

server:~# ls -alct /|tail -1|awk '{print $6, $7, $8}'
sep 6 2012

 

root home directory is created at install time
 

 

server:~# ls -alct /root

 

root@server:~# ls -lAhF /etc/hostname
-rw-r–r– 1 root root 8 sep  6  2012 /etc/hostname

 

For Debian / Ubuntu and other deb based distributions the /var/log/installer directory is being created during OS install, so on Debian the best way to check the Linux OS creation date is with:
 

root@server:~# ls -ld /var/log/installer
drwxr-xr-x 3 root root 4096 sep  6  2012 /var/log/installer/
root@server:~# ls -ld /lost+found
drwx—— 2 root root 16384 sep  6  2012 /lost+found/

 

On Red Hat / Fedora / CentOS, redhat based Linuces , you can use:

 

rpm -qi basesystem | grep "Install Date"

 

basesystem is the package containing basic Linux binaries many of which should not change, however in some cases if there are some security updates package might change so it is also good to check the root filesystem creation time and compare whether these two match.

Howto Fix “sysstat Cannot open /var/log/sysstat/sa no such file or directory” on Debian / Ubuntu Linux

Monday, February 15th, 2016

sysstast-no-such-file-or-directory-fix-Debian-Ubuntu-Linux-howto
I really love sysstat and as a console maniac I tend to install it on every server however by default there is some <b>sysstat</b> tuning once installed to make it work, for those unfamiliar with <i>sysstat</i> I warmly recommend to check, it here is in short the package description:<br /><br />
 

server:~# apt-cache show sysstat|grep -i desc -A 15
Description: system performance tools for Linux
 The sysstat package contains the following system performance tools:
  – sar: collects and reports system activity information;
  – iostat: reports CPU utilization and disk I/O statistics;
  – mpstat: reports global and per-processor statistics;
  – pidstat: reports statistics for Linux tasks (processes);
  – sadf: displays data collected by sar in various formats;
  – nfsiostat: reports I/O statistics for network filesystems;
  – cifsiostat: reports I/O statistics for CIFS filesystems.
 .
 The statistics reported by sar deal with I/O transfer rates,
 paging activity, process-related activities, interrupts,
 network activity, memory and swap space utilization, CPU
 utilization, kernel activities and TTY statistics, among
 others. Both UP and SMP machines are fully supported.
Homepage: http://pagesperso-orange.fr/sebastien.godard/

 

If you happen to install sysstat on a Debian / Ubuntu server with:

server:~# apt-get install –yes sysstat


, and you try to get some statistics with sar command but you get some ugly error output from:

 

server:~# sar Cannot open /var/log/sysstat/sa20: No such file or directory


And you wonder how to resolve it and to be able to have the server log in text databases periodically the nice sar stats load avarages – %idle, %iowait, %system, %nice, %user, then to FIX that Cannot open /var/log/sysstat/sa20: No such file or directory

You need to:

server:~# vim /etc/default/sysstat


By Default value you will find out sysstat stats it is disabled, e.g.:

ENABLED="false"

Switch the value to "true"

ENABLED="true"


Then restart sysstat init script with:

server:~# /etc/init.d/sysstat restart

However for those who prefer to do things from menu Ncurses interfaces and are not familiar with Vi Improved, the easiest way is to run dpkg reconfigure of the sysstat:

server:~# dpkg –reconfigure


sysstat-reconfigure-on-gnu-linux

 

root@server:/# sar
Linux 2.6.32-5-amd64 (pcfreak) 15.02.2016 _x86_64_ (2 CPU)

0,00,01 CPU %user %nice %system %iowait %steal %idle
0,15,01 all 24,32 0,54 3,10 0,62 0,00 71,42
1,15,01 all 18,69 0,53 2,10 0,48 0,00 78,20
10,05,01 all 22,13 0,54 2,81 0,51 0,00 74,01
10,15,01 all 17,14 0,53 2,44 0,40 0,00 79,49
10,25,01 all 24,03 0,63 2,93 0,45 0,00 71,97
10,35,01 all 18,88 0,54 2,44 1,08 0,00 77,07
10,45,01 all 25,60 0,54 3,33 0,74 0,00 69,79
10,55,01 all 36,78 0,78 4,44 0,89 0,00 57,10
16,05,01 all 27,10 0,54 3,43 1,14 0,00 67,79


Well that's it now sysstat error resolved, text reporting stats data works again, Hooray! 🙂

How to check if newly installed SSL certificate for IMAP and IMAPS is properly installed

Tuesday, June 28th, 2011

Did you have to regenerate your SSL certificate for your mail server’s IMAP and IMAP SSL service?
Did you have to find out if the newly installed certificates are fine after install?

Here is how:

           root@server-hosting [/usr/local ]# openssl s_client -connect imap.example.com:993
root@server-hosting [/usr/local ]# openssl s_client -connect imap.example.com:143 -starttls imap

The output returned by this two commands will be the imap and imaps configured certificates as well as extensive info concerning the installed SSL, the last chunk of info to be spit is most crucial to know if certificate is fine.
It should be something like:

...
New, TLSv1/SSLv3, Cipher is AES256-SHA
Server public key is 1024 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
Protocol : TLSv1
Cipher : AES256-SHA
Session-ID: 0B69E91022CB56D64F56CFA08405944D9C4C0069EE4097890B98F1406CF084D5
Session-ID-ctx:
Master-Key: 13745B94E0C5A0604EB7529E7409251961DFD5F4134F3A8F
Key-Arg : None
Start Time: 1309265383
Timeout : 300 (sec)
Verify return code: 18 (self signed certificate)
---
. OK CAPABILITY completed
closed

How to make sure your Linux system users won’t hide or delete their .bash_history / Securing .bash_history file – Protect Linux system users shell history

Monday, July 19th, 2010

linux-bin-bash-600x600logo
If you're running multi user login Linux system, you have probably realized that there are some clever users that prefer to prevent their command line executed commands to be logged in .bash_history.
To achieve that they use a number of generally known methodologist to prevent the Linux system from logging into their $HOME/.bash_history file (of course if running bash as a default user shell).
This though nice for the user is a real nightmare for the sysadmin, since he couldn't keep track of all system command events executed by users. For instance sometimes an unprivilegd user might be responsible for executing a malicious code which crashes or breaks your server.
This is especially unpleasent, because you will find your system crashed and if it's not some of the system services that causes the issue you won’t even be able to identify which of all the users is the malicious user account and respectively the code excecuted which fail the system to the ground.
In this post I will try to tell you a basic ways that some malevolent users might use to hide their bash history from the system administrator.
I will also discuss a few possible ways to assure your users .bash_history keeps intact and possibly the commands executed by your users gets logged in in their.
The most basic way that even an unexperienced shell user will apply if he wants to prevent his .bash_history from sys admins review would be of directly wiping out the .bash_history file from his login account or alternatively emptying it with commands like:

malicious-user@server:~$ rm -f. bash_history
ormalicious-user@server:~# cat /dev/null > ~/.bash_history

In order to prevent this type of attack against cleaning the .bash_history you can use the chattr command.
To counter attack this type of history tossing method you can set your malicious-user .bash_history’s file the (append only flag) with chattr like so:

root@server:~# cd /home/malicious-user/
root@server:~# chattr +a .bash_history

It’s also recommended that the immunable flag is placed to the file ~/.profile in user home

root@server:~# chattr +i ~/.profile

It would be probably also nice to take a look at all chattr command attributes since the command is like swiss army knife for the Linux admin:
Here is all available flags that can be passed to chattr
append only (a)
compressed (c)
don~@~Yt update atime (A)
synchronous directory updates (D)
synchronous updates (S)
data journalling (j)
no dump (d)
top of directory hierarchy (T)
no tail-merging (t)
secure deletion (s)
undeletable (u)
immutable (i)

It’s also nice that setting the “append only” flag in to the user .bash_history file prevents the user to link the .bash_history file to /dev/null like so:

malicious-user@server:~$ ln -sf /dev/null ~/.bash_history
ln: cannot remove `.bash_history': Operation not permitted

malicious-user@server:~$ echo > .bash_history
bash: .bash_history: Operation not permitted

However this will just make your .bash_history append only, so the user trying to execute cat /dev/null > .bash_history won’t be able to truncate the content of .bash_history.

Unfortunately he will yet be able to delete the file with rm so this type of securing your .bash_history file from being overwritten is does not completely guarantee you that user commands will get logged.
Also in order to prevent user to play tricks and escape the .bash_history logging by changing the default bash shell variables for HISTFILE an d HISTFILESIZE, exporting them either to a different file location or a null file size.
You have to put the following bash variables to be loaded in /etc/bash.bashrc or in /etc/profile
# #Prevent unset of histfile, /etc/profile
HISTFILE=~/.bash_history
HISTSIZE=10000
HISTFILESIZE=999999
# Don't let the users enter commands that are ignored# in the history file
HISTIGNORE=""
HISTCONTROL=""
readonly HISTFILE
readonly HISTSIZE
readonly HISTFILESIZE
readonly HISTIGNORE
readonly HISTCONTROL
export HISTFILE HISTSIZE HISTFILESIZE HISTIGNORE HISTCONTROL

everytime a user logs in to your Linux system the bash commands above will be set.
The above tip is directly taken from Securing debian howto which by the way is quite an interesting and nice reading for system administrators 🙂

If you want to apply an append only attribute to all user .bash_history to all your existing Linux server system users assuming the default users directory is /home in bash you can execute the following 1 liner shell code:

#Set .bash_history as attr +a
2. find /home/ -maxdepth 3|grep -i bash_history|while read line; do chattr +a "$line"; done

Though the above steps will stop some of the users to voluntary clean their .bash_history history files it won’t a 100% guaranttee that a good cracker won’t be able to come up with a way to get around the imposed .bash_history security measures.

One possible way to get around the user command history prevention restrictions for a user is to simply using another shell from the ones available on the system:
Here is an example:

malicious-user:~$ /bin/csh
malicious-user:~>

csh shell logs by default to the file .history

Also as far as I know it should be possible for a user to simply delete the .bash_history file overwritting all the .bash_history keep up attempts up-shown.
If you need a complete statistics about accounting you’d better take a look at The GNU Accounting Utilities

In Debian the GNU Accounting Utilities are available as a package called acct, so installation of acct on Debian is as simple as:

debian:~# apt-get install acct

I won’t get into much details about acct and would probably take a look at it in my future posts.
For complete .bash_history delete prevention maybe the best practice is to useg grsecurity (grsec)

Hopefully this article is gonna be a step further in tightening up your Server or Desktop Linux based system security and will also give you some insight on .bash_history files 🙂 .